Observations on Man

The first part of the text deals with the frame of the human body and mind and their mutual connections and influences, the second with the duty and expectations of mankind.

For example, "in Hartley's theory, emotion is a fluid like electricity or water"—it flows from one experience to the next, a concept he called transference and lifted from the writings of Gay.

[citation needed] Hartley believed that sensation is the result of vibrations of the minute particles of the medullary substance of the nerves, made possible by a subtle, elastic ether that was rare in the interstices of solid bodies and in their close neighbourhood.

Voluntary action is explained as the result of a firm connection between a motion and a sensation or "idea," and, on the physical side, between an "ideal" and a motory vibration.

Hartley's theory helped give birth to the modern study of the connection between the physiology of the human brain and "the mind".